The present invention relates to methods of preparing polycrystalline diamond compacts made by a high pressure-high temperature process (HP/HT) and has special application in the preparation of polycrystalline diamond compacts for use in wire dies.
A polycrystalline diamond compact comprises a mass of diamond-to-diamond bonded particles in which the diamond concentration is at least 70 volume percent. Representative wire drawing dies made with polycrystalline diamond compacts can be found, for example, in U.S. Pat. Nos. 3,831,428, 4,129,052, 4,144,739, 4,171,973, and 4,231,762. Such wire die compacts comprise an inner mass of polycrystalline diamond which inner mass is surrounded by and bonded to a mass of cemented metal carbide, such as cobalt cemented tungsten carbide. These are referred to as carbide-supported wire die compacts. Unsupported polycrystalline diamond wire die compacts without a surrounding mass of cemented metal carbide also are available. Carbide-supported polycrystalline diamond compacts in other configurations are described, for example, in U.S. Pat. Nos. 3,745,623, 4,108,614, 4,215,999, 4,219,339, 4,229,186, and 4,255,165.
The formation of diamond-to-diamond bonds between individual abrasive grains requires a catalyst/solvent (hereinafter referred to as a catalyst) that is able to wet the diamond surface as well as dissolve and precipitate diamond. This task normally is complicated by the presence of inhibiting impurities. The impurities can be in the form of surface-adsorbed species including oxygen and water, for example. The impurities can block pore passages between the grains and alter the diamond surfaces so that catalyst penetration becomes inadequate and surfaces are no longer wetted. The problems caused by impurities are magnified as the abrasive grain size decreases. Thus, decreasing pore size and increasing surface area to volume ratios are contributing factors that lead to a higher frequency of poorly bonded diamond compacts as smaller grains are used. However, the ease of finishing diamond compacts that have been sintered from fine grain diamond, e.g. less than about 10 microns, into a desired geometry make such compacts desirable in the marketplace. In the case of wire die compacts, the use of fine diamond grains enables an improvement in the surface finish of wires that have been drawn therethrough, making such compacts especially desirable. In addition to the noted affect of diamond particle size, it also has been observed that the incidence of flaws during fabrication generally tends to increase with overall compact size.
The high temperature/high pressure (HP/HT) process of sintering diamond into a coherent mass in making polycrystalline diamond compacts with a catalyst "sweep-through" technique as taught by U.S. Pat. Nos. 3,745,623 and 3,831,428 is well known in the art. An important feature of the sweep-through technique is that the amount of catalyst in the system is determined automatically by the available free volume within the mass of polycrystalline grains being sintered. The amount of catalyst is independent of diamond size, diamond size distribution, and the change in free volume during compression and sintering. Diamond bonding during the process is enhanced by the pushing of impurities from the diamond consolidation zone by the sweeping catalyst solvent front. In this regard, commonly assigned U.S. Pat. No. 4,525,179 discloses an improved sweep-through process which utilizes a pre-sweep of a relatively low melting point material, typified by copper, which preceeds a catalyst sweep through a diamond particle mass in the production of diamond compacts. Another recent commonly-assigned patent, U.S. Pat. No. 4,534,934 discloses an improved process in the manufacture of diamond wire die compacts in which catalyst sweeps both axially and radially into a diamond particle mass.
Another method suggested for decreasing the incidence of flaws, particularly in fine grain polycrystalline diamond compacts, is the addition of particles designed to inhibit excessive regrowth during sweep as proposed by Hara et al., "On the Properties of Fine Grain Sintered Diamond Bodies", Proceedings of the 10th Plainsee-Seminar, Hugo M. Ortner, Editor, Metal Work Plainsee, Reutte, Austria, Vol. 2, pp 581-589 (1981). Another technique proposed to improve the diamond compact portion of the wire drawing dies is the use of sintering aids as set forth in U.S. Pat. Nos. 3,913,280, 4,268,276, 4,370,149, and South African application No. 756730. Despite the many benefits which have been achieved in the art, the need exists for techniques which substantially enhance the reproduceability of well bonded polycrystalline diamond compacts, especially those sintered with fine grain diamond.